Coastal survey data for Perranporth Beach and Start Bay in southwest England (2006–2021)

Records of beach morphologic change and concurrent hydrodynamic forcing are needed to understand how coastlines in different environments change over time. This submission contains data for the period 2006 to 2021, for two contrasting macrotidal environments in southwest England: (i) cross-shore dominated, dissipative, sandy Perranporth Beach, Cornwall; and (ii) longshore-dominated, reflective gravel beaches within Start Bay, Devon. Data comprise monthly to annual beach profile surveys, annual merged topo-bathymetries, in addition to observed and numerically modelled wave and water levels. These data provide a valuable resource for modelling the behaviour of coastal types not covered by other currently available datasets.

www.nature.com/scientificdata www.nature.com/scientificdata/ orientated with the x-axis aligned positive offshore (coordinates also provided in OSGB36 British National Grid (BNG)). Beach volume over the patch (Fig. 2c), alongshore averaged to give volume per metre alongshore, is calculated as the surface integral above -2 m ODN. Volume change is taken relative to the benchmark initial survey. The volume time series shows a seasonal oscillation of 50-100 m 3  Cross-shore profiles. Cross-shore profiles spaced along Perranporth (Table 1, DS02) are collected by PCO, using pole-attached RTK-GNSS to spring low tide level (minimum depth -2 m ODN) at ten locations alongshore (lines 1-10 in Fig. 2a; see Table 2 for original PCO line names). Transects are 6-monthly at the southern end, and yearly at the north end. Cross-shore spacing between points is irregular, with more closely spaced points  Table 2 (Table 3) Multi-source (Table 3) OSGB36 BNG, ODN Table 1. Perranporth morphological datasets. *Local grid aligned to x-axis positive offshore.
www.nature.com/scientificdata www.nature.com/scientificdata/ around changes in slope. Data are provided at the original survey points, without interpolation. Transects cover the inter-and supratidal beach, as well as the dune system, and examples at three locations are shown in Fig. 3.
A full list of profile names used by Plymouth Coastal Observatory (PCO) are provided for Perranporth (Table 2). These can be used for referencing between the numbering used in Fig. 2 and the extended names used by PCO (https://southwest.coastalmonitoring.org/).  Table 2. Perranporth PCO transect names (south to north). Merged full embayment elevation model. An uncommon aspect of this data submission is the inclusion of digital elevation models (DEM) of the full nearshore, beach and dune systems ( Table 1, DS03) for the years 2011, 2016-2018 and 2021. These DEMS have been constructed using a range of gridded input datasets, outlined below and in Table 3. Timing of component dataset collection is given in Table 4. Greater detail is provided in Valiente et al. 29 .
Uncrewed aerial vehicle imagery. Drone-based, or Uncrewed Aerial Vehicle (UAV), photogrammetric data were collected for the dune area of the full length of the beach, using a DJI Phantom 4 (RTK) quadcopter for 2016-2018 and 2021. Coverage includes the supratidal to an elevation of >30 m ODN. For each flight 20-30 ground control points (GCPs) were distributed evenly throughout the survey volume (except for 2021 where RTK UAV used reduced GCP requirements). GCPs were surveyed using an RTK-GNSS Trimble 5800/R10. Images were processed using a Structure-from-Motion/Multi-View Stereo workflow (Agisoft MetaShape Pro) to produce a 1-m DEM, with vertical uncertainty (σ) of 0.04 m.
All-terrain vehicle surveys. All-Terrain Vehicle (ATV) mounted RTK-GNSS surveys were conducted over inter-tidal and supratidal (z = −2 to 4 m) for the full extent of the beach, employing a Trimble 5800/R10, running alongshore lines with cross-shore line spacing 20-25 m, for years 2016-2019 and 2021. ATV data were collected concurrently with single-beam echosounder (SBE) bathymetry, and combined to a merged data product (see next section). Mean uncertainty for ATV surveys (σ) is 0.05 m.     (Fig. 4) from the composite datasets described above. The method of DEM-generation involved an initial step of gridding the component surveys (Table 3), using a natural neighbour interpolation, and then merging these into one large composite DEM covering the entire embayment, including adjacent areas beyond the bounding inner headlands (Fig. 1b, green line). Extended methods are provided in ref. 29 . For 2011 (Table 4), the merged DEM was constructed with Lidar (EA) and MBE (UKHO). For subsequent years, the merged DEM was constructed using datasets collected by CPRG (UAV, ATV, SBE, MBE).
An example merged DEM product for 2018 ( Fig. 4a) indicates the merged survey product extent. Examples of morphologic change between surveys is demonstrated with elevation difference plots, or DEMs of Difference (DoD; Fig. 4b-e), showing contrasting beach response, including dominant beach erosion with subtidal accretion (Fig. 4b,e); dominant beach recovery with subtidal lowering (Fig. 4c); and a mix of erosion and accretion across the intertidal to shallow subtidal (Fig. 4d). Detailed interpretation of full embayment morphologic change is provided in ref. 29 .
Due to the difficulties in obtaining complex, multi-method survey data, there are instances where the constituent datasets for the merged DEM were obtained over different months or years. This introduces a degree of uncertainty, as bed level change may occur between surveys. Additionally with merged DEMs, there may be offsets between datasets that may represent either measurement error and/or morphologic change between surveys. These sources of uncertainty are acknowledged, though are inherently difficult to quantify. Best-practice standards have been followed in determining variable uncertainty maps 29,38 . Where there were overlapping data, priority was given to the most reliable dataset (in time and uncertainty levels). The greatest time mismatches occur for surveys prior to the 2013/14 storm season (e.g., Perranporth, 2011 DEM, Table 4). Here, the assumption was made that the changes occurring over that extreme winter season 21-23 would be an order of magnitude larger than bed change occurring between the component surveys. In addition, the greatest proportion of morphological change occurs over the shallow bar system (captured by SBE) and the intertidal and supra tidal beach. These critical surveys have minimal temporal mismatch. Greatest mismatch is with MBE data, typically used in areas of >15 m water depth, where less change is expected. Methods for validating all survey data, including determination of offsets between surveys, are described in the "Technical Validation" section. For users calculating difference between DEMs, it is recommended to apply thresholding based on combined uncertainty 38 .
Perranporth waves and water level. Wave and water level data are provided from observations and regional numerical models (Table 5; Fig. 5).
Wave observations. Wave observations ( Table 5,   www.nature.com/scientificdata www.nature.com/scientificdata/ height, peak and zero crossing wave period, peak direction, directional spread) and sea surface temperature. Gaps are present for some storm events, which may be filled using the included modelled wave data (see below). Metadata reports obtained through PCO are included with the dataset. Water level observations. Water level observations (Table 5, DS05; Fig. 5c) were obtained from the PCO data portal (https://southwest.coastalmonitoring.org/) for the nearest available tide gauge, an Etrometa Step Gauge, at Port Isaac, 35 km northeast of Perranporth (Lon −4.83338°, Lat 50.59518°). Variables include water level (Ordnance Datum Newlyn and Chart Datum) and residual (difference in measured water level from tidal prediction), recorded at 10-min intervals for 2006 -2021. It is not recommended that the observed Port Isaac water levels be used directly for Perranporth, as there are significant water level variations in tidal range and timing of high/low water between these points, due to a steep tidal range gradient in the region. However, the Port Isaac tidal gauge data are included as they provide the nearest available observations of water level residual, and may be useful for numerical model validation.
Wave model output. Numerically modelled wave conditions (  www.nature.com/scientificdata www.nature.com/scientificdata/ produced by the UK Met Office, at c. 1.5 km grid resolution, with a 3-hourly timestep, for 1980-present. Included with this data submission is a single wave model node offshore of south Perranporth Beach ( Fig. 1b; Lon −5.1986°, Lat 50.3441°; c. 26 m water depth) for 2006-2021, inclusive. Variables include spectral wave statistics (wave height, period and direction), with a full list available through the CMEMS data portal. A user manual and data quality report are included with the dataset.
A statistical comparison between the wave model node and wave buoy observations was conducted, examining mean bias (positive result indicates a higher value for the model node), and Root Mean Square (RMS) difference. Results include H s (bias = −0.03 m, RMS = 0.25 m); T p (bias = −0.01 s; RMS = 1.8 s); and peak direction D p (bias = 6.7°; RMS = 15°). Overall there is good agreement between observations and model, noting the output points are not co-located (Fig. 1b), therefore differences in wave statistics, for direction in particular, may be expected.
Hydrodynamic model output. Numerically modelled water levels for Perranporth (Table 5,

Methods Start Bay
Study site. Start Bay is a 12-km long embayment aligned SSW-NNE and located on the south coast of Devon, SW England (Fig. 1a). The embayment consists of four interconnected gravel barriers, backed by freshwater lagoons or marshes, and separated at high tide by protruding rocky headlands and shore platforms. From south to north, these gravel beaches are Hallsands, Beesands, Slapton Sands and Blackpool Sands. Of the four gravel beaches, Slapton Sands is the largest and most intensely studied (cf. "Background and Summary" section). The beach is 3.5 km long and the barrier is up to 120 m wide and rises to 6-8 m ODN from south to north. A large freshwater lake, called Slapton Ley (Fig. 1d,e), with a water level close to the ocean high tide level 45 , lies behind the barrier. The high-tide beach at Slapton Sands is only 10-20 m wide at the seawall-backed southern extremity, but is more than 100 m wide at its northern end. Sediment size is highly variable and the median sediment size D 50 is 2-10 mm, with sediment size increasing from south to north 46 . On all beaches, the beachface is steep (tanβ = 0.125) and the transition to a low-gradient sandy bottom occurs around a depth of 8-10 m depth 38,47 , relative to ODN.
The bay is impacted by a bi-modal wave climate (Fig. 1a), with a dominant component of southerly waves and less frequent easterly waves 35 . The easterly waves are locally generated in the English Channel, but the southerly waves generally refract into the Channel from the Atlantic from an initial westerly direction. The wave climate is strongly seasonal, with summer and winter significant wave heights of H s = 0.5-0.6 m and H s = 1-1.3 m, respectively 38 . Maximum wave heights during storms in Start Bay can attain H s = 5 m. These extreme waves occur less frequently than once a year, and may arrive from the east (e.g., 'Beast from the East' event in 2018) or the south (e.g., Atlantic storms during the 2013/14 extreme winter). The tidal regime is macro-tidal with a spring and neap tidal range of 4.3 m and 1.8 m, respectively. The tidal water motion in Start Bay can be described as a large scale anti-clockwise eddy 39 , where the eddy is at the same time the result of and the cause for the large subtidal banner bank, called Skerries, located in the southwest part of the bay. Skerries comprises medium shelly sand 48 and extends across almost half of Start Bay with the crest only a few meters below low tide level; therefore, it exerts a significant influence on the inshore wave climate and affects both the wave height and direction along the coast, especially for waves from the south 39 .
The southerly and easterly wave directions drive northward and southward sediment transport, respectively, and the beaches in the embayment are continually in a state of dynamic equilibrium, with the planform shape rotating in response to the current wave approach 23 . The Start Bay embayment as a whole is a closed system 38 , bounded by significant northern and southern headlands; however, beach rotation and exchange of sediment between the individual sub-embayments occur through headland bypassing under extreme wave conditions 37 and sustained periods of a particular wave direction 38 . Based on a total sediment budget approach, the northward sediment transport during the 2013/14 extreme winter along Slapton Sands is estimated 38 at 500,000 m 3 , while the southward sediment transport during one of the most energetic easterly storms in 2018 is estimated 37 at 200,000 m 3 .
A range of human interventions are located across Start Bay; with some areas heavily engineered, and other sections in a more natural state. The positioning of protection structures toward the southern end of the southern sub-embayments reflects the long-term trend of northward sediment transport and clockwise rotation. At the far southern end of the bay (500 m south of "HS" in Fig. 6a), the abandoned village of Old Hallsands lies in ruin, destroyed by storms after persistent erosion, possibly related to shifts in wave climate, and nearshore dredging 36 . The southern end of Hallsands is backed by a rock armour revetment, in poor condition. The southern third of Beesands features compound rock armour and seawall, protecting a small number of residential and commercial buildings. A short section of rock gabions is present at north Beesands. Torcross, at the southern end of Slapton Sands is heavily protected by compound revetment and seawall, protecting properties behind the wall, extending northward into a rock revetment protecting the road along the crest of the barrier. The middle section of Slapton Sands, comprising a narrow dune backed by the road, has seen a number of damaging storm events, with a 2018 event destroying a section of road and carpark 37 . The northern section of Slapton Sands is cliffed-backed and has minimal interventions, while at the far north of the bay, Blackpool Sands is largely unprotected apart from a small section of wall fronting a retail premises behind the mid-point of the beach. (2023) 10:258 | https://doi.org/10.1038/s41597-023-02131-0 www.nature.com/scientificdata www.nature.com/scientificdata/ Start Bay morphological data. The structure of the morphological data for Start Bay (Table 6) comprises: (1) beach-dune transects, conducted monthly for Slapton Sands [CPRG, PCO] and 6-to 12-monthly for the other beaches [PCO; Hallsands, Beesands, Blackpool Sands]; and (2) annual full embayment surveys, covering the alongshore extent of all four beaches, from onshore of the barrier to beyond the 10 m depth contour (ODN). The full data collection programme and an analysis of the complete dataset are presented in Wiggins et al. 38 ; this submission provides a subset of those data.
Cross-shore profiles. Cross-shore transects for Start Bay (Table 6, DS08) comprise surveys collected by CPRG and PCO (Fig. 6). All surveys were conducted on foot using RTK-GNSS during spring tide, with surveys generally extending from onshore of the barrier crest (where accessible) down to near spring low tide level (−1 to −2 m ODN). CPRG transects at Start Bay include 21 lines covering Slapton Sands at c. 250-m alongshore spacing (Fig. 6c), surveyed monthly from 2007 to present. Some lines have intermittent coverage, or substantial time gaps. Example CPRG profiles and beach volume time series at opposite ends of Slapton Sands (Fig. 7) show a trend of clockwise rotation, with erosion from the southern end ( Fig. 7; P1) and accretion at the northern end (P18). Detailed methods are provided in ref. 35 .
PCO transects at Start Bay include 37 lines covering the extent of Hallsands, Beesands, Slapton Sands and Blackpool Sands, with varying alongshore spacing (Fig. 6), surveyed from 2007 to present. Modal survey frequency is 6-monthly, with some gaps, occasional post-storm surveys, and some periods with more frequent surveys (e.g., up to six per year in 2017/2018). Note that Slapton Sands is covered by both CPRG and PCO, using two separate (non-aligned) sets of profile lines (Fig. 6c) (Table 8) Multi-source (Table 8) OSGB36 BNG, ODN Table 6. Start Bay morphological datasets.

Fig. 6 Profile locations across Start Bay, including Hallsands (HS), Beesands (SS), Slapton Sands (SS) and
Blackpool Sands (BK). CPRG profiles cover only Slapton Sands and are labelled "P00" to "P20" (c; orange labels). PCO profiles cover all beaches and are abbreviated ("HS_", "BS_", "SL_", "BK_"). Full PCO profile IDs are listed in Table 7. www.nature.com/scientificdata www.nature.com/scientificdata/ from each of the four beaches are provided in Fig. 8, showing a trend of erosion for the southern beaches (Hallsands, Beesands; Fig. 8a,b), and substantial accretion at the northern end of the bay (Blackpool Sands; Fig. 8d). Additional metadata and baseline survey data are available through PCO (https://southwest.coastalmonitoring.org/). Both CPRG and PCO profiles are sampled at irregular distances cross-shore, with higher resolution around changes in slope; data are provided at the original sample points, without interpolation.
A full list of profile names used by Plymouth Coastal Observatory (PCO) are provided for Start Bay (Table 7). These can be used for referencing between the abbreviated names used in Fig. 6 and Fig. 8, and the extended names used by PCO (https://southwest.coastalmonitoring.org/).
Merged full embayment elevation model. Full embayment DEMs with grid resolution 1-m are provided for Start Bay (Table 6, (Tables 8, 9; Fig. 9). Merged DEMs have been constructed using the same survey methods as for Perranporth, including UAV, Lidar and MBE. For these methods, refer to "Merged full embayment elevation model" sub-heading in Perranporth methods, and ref. 38 . MBE bathymetric surveys for Start Bay were obtained from UKHO for the 2013 epoch and by CPRG for subsequent years. Additional to these methods, the Start Bay merged DEMs include isolated areas of pole-mounted RTK-GNSS coverage, obtained by CPRG, typically used in areas where UAV flights were not permitted. In this instance, full coverage was achieved by having a surveyor walk closely-spaced (c. 5-m) alongshore lines 38 . A sample Start Bay full embayment survey is provided for 2018 (Fig. 9a), indicating alongshore and cross-shore extent. Example difference DEMs are included for the 2013-2018 epoch, encompassing a period of significant southwest to northeast sediment transport along the extent of Start Bay (i.e., clockwise rotation), capturing erosion around Beesands at the southern end of the bay (Fig. 9b) and accretion around Blackpool Sands at the northern end (Fig. 9c).

Start Bay waves and water level.
Waves and water levels for Start Bay are provided using a combination of observations and regional numerical model observations, summarised in Table 10 and Fig. 10. The data sources, equipment, and methodology are as per those for Perranporth (refer to section "Perranporth waves and water level" for detailed methods), including wave buoy and tide gauge observations obtained through PCO (https://southwest.coastalmonitoring.org/), and numerical modelling data via CMEMS (https://marine.copernicus.eu). Extended data sets may be freely accessed via these portals.
Wave buoy data for Start Bay (Table 10, DS10; Fig. 10a) were collected by a Datawell Waverider III buoy moored at c. −16 m ODN directly offshore the NE end of Slapton Sands (Fig. 1d; mean location Lon −3.6162°, Lat 50.2918°). Water levels (Table 10,  They are provided as they include observed water level residual, and may be used for validation of hydrodynamic models. Wave model outputs (Table 10,    www.nature.com/scientificdata www.nature.com/scientificdata/ that captures both the southwesterly and easterly wave directions. Hydrodynamic model outputs, including water level (Table 10,  A statistical comparison between the Start Bay wave model node and wave buoy was conducted, using the methods previously described for Perranporth, determining values for H s (bias = 0.11 m, RMS = 0.28 m); T p (bias = −0.67 s; RMS = 3.6 s); and D p (bias = 29°; RMS = 31°). Agreement between wave model and observations is reasonably good, noting the model node and wave buoy are not co-located, and also that wave direction may be more reliably predicted during high wave events 37 .    www.nature.com/scientificdata www.nature.com/scientificdata/

Data Records
The full data record is available through an open access data repository 49 . A summary of all 13 datasets is provided in Table 11; this includes seven datasets for Perranporth (three morphological, two wave and two water level) and six datasets for Start Bay (two morphological, two wave and two water level). For each dataset, reference is provided to the relevant text section where detailed methodological information is provided. Data span the period of 2006-2021, and all datasets were ongoing at the time of publication, with the exception of the Full Embayment DEMs (dataset IDs 03, 09), for which there may be intermittent updates in future.
This submission represents a comprehensive long-term hydro-morphodynamic data for a macro-tidal beach (Perranporth) and a gravel beach (Start Bay). Furthermore, these are amongst the few available datasets containing time series of full embayment DEMs, which capture the entire active zone of sediment transport (to the depth of closure) at both sites. For these reasons, these data are highly valuable in modelling how coastlines such as these will respond to future changes in sea level and wave climate.  Topographic surveying methods (UAV and RTK-GNSS) were compared against a high-precision survey reference surface, obtained by surveying a section of beach with a Leica terrestrial laser scanner, using reference control points measured with a total station. This method accounts for total uncertainty and/or bias for each survey method. UAV survey comparison with the reference surface resulted in a mean difference (bias) of 0.02 m and root-mean-square-error (RMSE) of 0.04 m. The RTK-GNSS recorded a mean difference of −0.008 m and RMSE of 0.05 m (details in ref. 38 ).
Sub-tidal survey uncertainty (random error) was determined for MBE surveying by applying a combined statistical and error budget modelling approach, based on prior estimates of uncertainty and total propagated uncertainty for each sounding. The uncertainty estimates were gridded using the Combined Uncertainty and Bathymetric Estimator (CUBE) algorithm, commonly used for generating spatially variable uncertainty 44 . As no absolute control surface was available to assess error across bathymetry surveys, a reference surface was surveyed across a 50 m by 50 m region of flat, rocky seabed at ~15 m depth to assess potential systematic error across years. Reference surface analysis for both sites is provided in Table 12, taking 2017 as a reference year for Perranporth, and 2021 for Start Bay. CUBE uncertainty was variable across the grids with typical uncertainty (σ) range of 0.01 to 0.3 m. Values for each survey method are shown in Tables 3, 8 for Perranporth and Start Bay, respectively.
Validation and quality control of third-party datasets. Wave and water level observations were obtained through the Plymouth Coastal Observatory (PCO). Documentation on quality control is available through PCO (https://southwest.coastalmonitoring.org/). Wave data accuracy (uncertainty) is reported as wave height (3%) and wave direction (1.5 degrees). Water level uncertainty from tide gauges is reported as 0.01 m.
Wave and hydrodynamic modelling outputs were obtained through the Copernicus Marine Environment Monitoring Service (CMEMS; https://marine.copernicus.eu). Technical reports on validation of the wave model 50 Table 11. Summary of all included datasets. * Data file names concatenate ID and Dataset name, and are contained in a ZIP file, e.g., "DS01_PPT_Beach3D.zip".